If I rack my lager Beer after 3 Weeks of Primary and two days of Diacetyl rest , would it be in danger of oxidation , as the Beer Volume is 4 liters and I'm going to keep it for 6 Weeks in a 5-liter-Carboy and the Beer won't emit CO2 anymore ?!

In my opinion - don't worry about it. You're putting an airlock on it, there's plenty of CO2 dissolved in the beer and some more that will doubtless be generated soon after you rack it. How much oxidation could there be?

Some brewers will purge there carboy with co2 before racking into it. Others go as far as to purge there transfer hoses with co2 as well. There will be some oxygenation but such a small amount most palates will not be able to taste it.